Power transmission device for electric vehicle

ABSTRACT

A power transmission device for a vehicle is comprised of a motor including a rotor shaft and a stator having an electromagnetic coil; an inverter configured to generate an alternating current, the inverter being connected with the coil to controllably rotate the rotor shaft relative to the stator; a gear set including an input shaft coupled with and rotated by the rotor shaft, an output shaft and gears so meshed as to transmit torque of the input shaft to the output shaft; and a grounding path electrically connecting a part of the gear set with a body of the vehicle, the part being so disposed as to have the input shaft electrically interposed between the part and the rotor shaft.

TECHNICAL FIELD

The present invention relates to power transmission devices applicable to electric vehicles with measures for preventing radiation of high-frequency noise.

BACKGROUND ART

It is known that, in an electric vehicle equipped with a motor as a driving power source, an inverter for controlling the motor generates high-frequency noise (or referred to as “radio noise”) and a motor drive system including a power transmissions, suspensions and such can be antennas to radiate the high-frequency noise, which cause negative effect on radio reception.

The following document discloses a related art, which proposes having a brush in electrical contact with an output shaft of a motor and grounding the brush via a vehicle body.

CITATION LIST Patent Literature

PTL 1: Japanese Patent Unexamined Application Laid-open No. 2006-320129

SUMMARY OF INVENTION Technical Problem

In the aforementioned related art, it is expected that the output shaft along with the motor is successfully grounded via a brush and thereby noise propagation is prevented. However, the present inventors have found out that the noise further leaks out of the output shaft toward a power transmission system and suspensions and is there radiated outward.

The present inventors have further studied noise propagation in the vehicle body and have found out that an electric resistance at the brush is not sufficiently low and therefore the noise can flow beyond a part with which the brush contacts. To reduce noise propagation, it is promising to reduce the resistance at the brush. However, reduction in electric resistance at the brush is not readily achieved.

The present invention has been achieved in view of the aforementioned problem. An object of the present invention is to provide a power transmission device applicable to electric vehicles, which suppresses noise propagation from an inverter through a power transmission to drive shafts and suspensions, thereby reducing noise radiation to the exterior.

Solution to Problem

According to an aspect of the present invention, a power transmission device for a vehicle is comprised of a motor including a rotor shaft and a stator having an electro-magnetic coil; an inverter configured to generate an alternating current, the inverter being connected with the coil to controllably rotate the rotor shaft relative to the stator; a gear set including an input shaft coupled with and rotated by the rotor shaft, an output shaft and gears so meshed as to transmit torque of the input shaft to the output shaft; and a grounding path electrically connecting a part of the gear set with a body of the vehicle, the part being so disposed as to have the input shaft electrically interposed between the part and the rotor shaft.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of an electric vehicle with a power transmission device according to a first embodiment of the present invention, which in particular shows a relation among the power transmission device, wheels and the ground.

FIG. 2 is an enlarged cross sectional view of the power transmission device around brushes.

FIG. 3 is a schematic drawing of an electric vehicle without measures for high-frequency noise, which schematically shows propagation pathways of the noise.

FIG. 4 is a schematic drawing of an equivalent circuit with respect to the electric vehicle with the power transmission device according to the first embodiment.

FIG. 5A demonstrates noise suppression when the electric vehicle runs at 5 km/h.

FIG. 5B demonstrates noise suppression when the electric vehicle runs at 40 km/h.

FIG. 6 is a schematic view of an electric vehicle with a power transmission device according to a second embodiment of the present invention.

FIG. 7 is a schematic view of an electric vehicle with a power transmission device according to a third embodiment of the present invention.

FIG. 8A shows a planetary gear used in the power transmission device according to the third embodiment.

FIG. 8B shows revolutions of respective gear members of the planetary gear.

FIG. 9 is a schematic view of a resistor and related members according to a modified embodiment.

DESCRIPTION OF EMBODIMENTS

Exemplary embodiments of the present invention will be described hereinafter with reference to the appended drawings.

Referring to FIG. 1, a power transmission device for an electric vehicle according to a first embodiment of the present invention will be described.

The power transmission device of the first embodiment is comprised of a motor 1, an inverter 2, a first gear set 3 including a plurality of couplings and meshing parts 4 functioning as resistors, and a grounding path 5.

The motor 1 is comprised of a motor housing 1 a, a rotor shaft 1 b rotatably supported by the motor housing 1 a, a rotor 1 c drivingly coupled with the rotor shaft 1 b, and stators 1 e fixed to the motor housing 1 a. Each stator 1 e has an electric coil 1 d coiled therearound. The rotor 1 c is accordingly comprised of a plurality of permanent magnets, thereby being rotated by a magnetic field generated by the coils 1 d. Specifically, any permanent magnet synchronous motor may be applied to the motor 1.

The inverter 2 is connected with the motor coils 1 d of the motor 1 via a three-phase power cable 6, and is in turn connected with a secondary battery 7 via a power cable 8. The inverter 2 includes switching devices to convert direct current supplied by the battery 7 into alternating current and further convert alternating current from the motor 1 into direct current. Thus the inverter 2 generates alternating current to drive the motor 1 when the vehicle gathers speed, and further regenerates electric power out of the kinetic energy of the vehicle when the vehicle decelerates.

The first gear set 3 is a reduction gear set and intervenes between a rotor shaft 1 b and output shafts 3 d respectively coupled with wheels 9, thereby transmitting torque from the rotor shaft 1 b to the output shafts 3 d with reducing its speed. Of course, the first gear set 3 may be a speed-up gear set or an isokinetic gear set.

The first gear set 3 includes a combination of shafts and gears, all of which are in general carried by a gear casing 3 a. The shafts include an input shaft 3 b drivingly coupled with and rotated by the rotor shaft 1 b, a counter shaft 3 c, and output shafts 3 d for driving wheels 9 in general. The gears include an input gear 3 e on the shaft 3 b, a first counter gear 3 f on the shaft 3 c meshing with the input gear 3 e, a second counter gear 3 g also on the shaft 3 c, and a drive gear 3 h on the output shafts 3 d meshing with the second counter gear 3 g. To allow differential motion between the right and left wheels, a differential gear 3 i intervenes between the drive gear 3 h and the output shafts 3 d. Thus the gears are so meshed as to transmit torque of the input shaft 3 b to the output shafts 3 d. Of course, the gear set 3 may be comprised of a smaller or greater number of shafts and gears.

As will be understood from the above description, the gear set 3 includes a plurality of meshing parts, namely a meshing part 4 d at which the gear 3 e meshes with the gear 3 f, and a meshing part 4 e at which the gear 3 g meshes with the gear 3 h. These meshing parts 4 d and 4 e inherently have considerably greater electric resistances than those of the shafts and gears, thereby functioning as resistors. Further, while the gear set 3 may further have a spline coupling 4 a for coupling with the rotor shaft 1 b, and output couplings 4 b, 4 c for respectively coupling with drive shafts 3 d′, these couplings 4 a, 4 b, 4 c also function as resistors.

The grounding path 5 includes one or more brushes 5 a, one or more lead lines 5 e and a connection line 5 b. The brushes 5 a are kept in slidable contact with, and are therefore electrically connected with, an end portion 3 c′ of the counter shaft 3 c. The lead lines 5 e are electrically connected with both the brushes 5 a and the gear casing 3 a. The connection line 5 b establishes electric connection of the gear casing 3 a with a body 10 of the vehicle. Thus the brushes 5 a, the lead lines 5 e and the connection line 5 b in combination with the gear casing 3 a constitute the grounding path 5 for electrically grounding a part of the first gear set 3. Meanwhile, in FIG. 1, left and right suspensions 11 are further illustrated.

Details of the brushes 5 a and related parts will be further described with reference to FIG. 2, hereinafter.

The counter shaft 3 c is rotatably supported by the gear casing 3 a with having a ball bearing 13 interposed therebetween. The counter shaft 3 c is, within a gear chamber 3 j inside the casing 3 a, splined to drivingly engage with the first counter gear 3 f. At an opposite side relative to the bearing 13, an oil seal 14 is provided so as to seal oil within the casing 3 a. Any anti-displacement means such as a snap ring 15 is secured to the end portion 3 c′ of the counter shaft 3 c. The end portion 3 c′ is preferably further elongated outward, where the brushes 5 a, preferably provided in pair, are kept in contact with the counter shaft 3 c.

A cover 16 is attached to the casing 3 a so as to cover the brushes 5 a and the end portion 3 c′. The cover 16 and the gear casing 3 a enclose a brush chamber 17 and the cover 16 is preferably comprised of a breather connector 12 to assure air ventilation.

Within the brush chamber 17, a brush case 5 c of an electrically non-conductive material is secured to the gear casing 3 a by means of an elongated portion 3 a′ of the casing 3 a with a securing means such as a bolt. Each brush 5 a along with a compressed spring 5 d is housed in the brush case 5 c and is thus urged toward the end portion 3 c′ of the counter shaft 3 c. The lead lines 5 e connected with the brushes 5 a are led out of the brush case 5 c and are electrically connected with the gear casing 3 a.

How the present embodiment suppresses noise radiation will be described hereinafter.

As described above, it is known that, in an electric vehicle or a so-called hybrid vehicle, an inverter for controlling a motor generates high-frequency noise and a motor drive system including a power transmission system and suspensions can be antennas to radiate the high-frequency noise, which causes negative effect on radio reception.

FIG. 3 schematically illustrates such noise radiation. An inverter includes switching devices which repeat turn on and off current flow, thereby generating alternating current. Each time the switching devices turn on current flow, sharp rising edges occur in the current flow and in turn cause high-frequency noise. The high-frequency noise, as illustrated by thick lines in FIG. 3, flows through a power cable PC and a motor coil MC to a rotor shaft MS of a motor M. Without any measure for high-frequency noise, the noise further flows through an input gear IG, a counter shaft CS, a differential gear DG and drive shafts DS to suspensions S of the vehicle. These noise conduction paths as a whole function as an antenna for radiating the noise to the exterior.

If a brush is provided and grounded, an equivalent circuit can be drawn in a way as shown in FIG. 4. An output voltage V2 of the noise present at a connection point CP where the brush contacts with any part of the power transmission device is represented by an equation:

V2={ZB/(ZB+ZR)}×V1−(1),

where V1 represents a voltage of the noise present at the rotor shaft 1 b, ZR represents a resistance present between the shaft 1 b and the connection point CP, and ZB represents a resistance through the brush toward the ground.

The lower the voltage V2 is, the less the noise current flows beyond the connection point toward the drive shaft DS. Thus suppression of the voltage V2 is required in order to suppress noise radiation.

As will be understood from the above equation (1), both increase in the resistance ZR and decrease in the resistance ZB are effective in suppression of the voltage V2 at the connection point CP, which leads to suppression of noise radiation as described above.

If a brush contacts with a rotor shaft as with the art disclosed in the PTL 1, substantially there is not a resistive element between the rotor shaft 1 b and the connection point CP. Thus the resistance ZR is relatively low and the resistance ZB is relatively high. A considerable amount of noise may flow beyond the connection point CP toward the drive shaft DS, where the noise is radiated.

In contrast, according to the present embodiment, as the brushes 5 a are kept in electrical contact with the end portion 3 c′ of the counter shaft 3 c, the coupling 4 a and the meshing part 4 d are electrically interposed between the rotor shaft 1 b and the part where the grounded brushes 5 a electrically contacts with the counter shaft 3 c. The coupling 4 a and the meshing part 4 d have considerably high resistance as described earlier. Thus the resistance ZR is made relatively high, thereby reducing the noise voltage V2 and suppressing the noise current flowing beyond the connection point CP.

The brushes 5 a may be disposed in any location in the first gear set 3 as far as one or more electrically resistive elements are electrically interposed between the rotor shaft 1 b and a part where the grounded brushes 5 a electrically contacts with the first gear set 3. As the resistive elements, meshing parts between any meshing gears, couplings, and any resistor connected in series in a shaft or a gear can be exemplified. This structure effectively suppresses noise radiation.

The effect of noise suppression is further demonstrated in FIGS. 5A and 5B. Each figure compares three cases of; (A) the rotor shaft 1 b is grounded at a location A shown in FIG. 1; (B) the input shaft 3 b is grounded at a location B; and (C) the counter shaft 3 c is grounded at a location C (the present embodiment). FIG. 5A shows cases where a vehicle runs at 5 km/h, and FIG. 5B shows case where a vehicle runs at 40 km/h. Each axis of ordinate represents radiated noise intensities measured in decibels relative to a standard intensity.

Both FIGS. 5A and 5 b illustrate that the case (A) (the rotor shaft 1 b is grounded) provides the greatest noise intensities. In the case (B) (the input shaft 3 b is grounded) as compared with the case (A), the noise intensities are suppressed. In the case (C) (the counter shaft 3 c is grounded), the noise intensities are further suppressed.

As well as the aforementioned effects, the present embodiment further provides the following effects.

The oil seal 14 intervenes between the gear chamber 3 j in which lubricant oil circulates and the brush chamber 17 which houses the brushes 5 a, thereby separating these chambers. Thus the oil does not ill-affect electrical continuity between the brushes 5 a and the counter shaft 3 c, and further fragments rubbed off from the brushes 5 a or the counter shaft 3 c do not ill-affect lubrication in the gear chamber 3 j.

As the breather 12 establishes ventilation between the brush chamber 17 and the exterior, pressure difference therebetween is effectively prevented. This results in prevention of oil leakage to the brush chamber 17, which may be driven by differential pressure.

In the aforementioned first embodiment, the reduction gear with three shafts is used.

A gear set with four or more shafts may be alternatively used. FIG. 6 illustrates a second embodiment in which an electric vehicle includes a second gear set 23 of a reduction gear set with four shafts.

The second gear set 23 intervenes between a rotor shaft 1 b and output shafts 23 e coupled with wheels 9, thereby transmitting torque from the rotor shaft 1 b to the output shafts 23 e with reducing its speed.

The second gear set 23 includes a combination of shafts and meshing gears, all of which are in general carried by a gear casing 23 a. The shafts include an input shaft 23 b drivingly coupled with and rotated by the rotor shaft 1 b, a first counter shaft 23 c, a second counter shaft 23 d and output shafts 23 e for driving wheels 9 in general. The gears include an input gear 23 f on the input shaft 23 b, first counter gears 23 g, 23 h on the first counter shaft 23 c, one of which meshes with the input gear 23 f, second counter gears 23 i, 23 j on the second counter shaft 23 d, one of which meshes with the first counter gear 23 h, and a drive gear 23 h on the output shafts 23 e meshing with the second counter gear 23 j. To allow differential motion between the right and left wheels, a differential gear 23 m intervenes between the drive gear 3 h and the output shafts 23 e. Thus the gears are so meshed as to transmit torque of the input shaft 23 b to the output shafts 23 e. Of course, the gear set 23 may be comprised of a smaller or greater number of shafts and gears.

The gear set 23 includes a spline coupling 4 a, meshing parts 4 d, 4 e, 4 f where gears 23 f, 23 g, 23 h, 23 i, 23 j, 23 k mesh with each other, and output couplings 4 b, 4 c, all of which are electrically resistive.

A grounding path 5 includes one or more brushes 5 a in slidable contact with an end portion 23 d′ of the second counter shaft 23 d. The brushes 5 a are grounded through the gear casing 3 a and the connection line 5 b, thereby grounding the second counter shaft 23 d. Of course, location of the brushes 5 a may be modified so as to electrically ground any other shaft 23 b, 23 c or 23 e.

The second embodiment provides the same effects as those of the first embodiment.

Alternatively, epicyclic gearing or planetary gearing may be used instead of the aforementioned reduction gears of a parallel shaft system. FIG. 7 illustrates a third embodiment in which a gear set 33 with planetary gearing is used.

The power transmission device of the third embodiment is comprised of a motor 1, an inverter 2, the gear set 33 including electrically resistive elements 4, and a grounding path 5. Aside from the gear set 33, those as described earlier can be applied to these components 1, 2 and 5 and therefore detailed descriptions thereof will be omitted.

The gear set 33 intervenes between a rotor shaft 1 b of the motor 1 and output shafts 33 d, thereby transmitting torque from the rotor shaft 1 b to the output shafts 33 d.

The gear set 33 includes a combination of shafts and meshing gears, all of which are in general carried by a gear casing 33 a. The shafts include an input shaft 33 b, a cylindrical shaft 33 c coaxial with the input shaft 33 b and an output shaft 33 d. The gears include a planetary gear set of a single pinion type, which is comprised of a sun gear 33 e, pinions 33 f meshing with and revolving about the sun gear 33 e, a ring gear 33 g meshing with the pinions 33 f, and an output gear 33 h for output. A pinion carrier 33 k holds the pinions 33 f and the cylindrical shaft 33 c is secured thereto. The gear set 33 further includes a differential gear having a drive gear 33 i meshing with the output gear 33 h.

The ring gear 33 g is secured to gear casing 33 a by means of a connection member 5 f as shown in FIGS. 7 and 8A. Therefore, when the sun gear 33 e rotates, the pinions 33 f along with the pinion carrier 33 k revolves with reduced revolution relative to the sun gear 33 e on the basis of the stationary ring gear 33 g as shown in FIG. 8B. Thus torque with reduced speed is output to the output gear 33 h and further transmitted to the output shafts 33 d via the differential gear.

The connection member 5 f also establishes electric connection between the ring gear 33 g and the gear casing 33 a. A connection line 5 b electrically connects the gear casing 33 a with the body 10 of the vehicle. Thus the connection member 5 f, the gear casing 33 a and the connection line 5 b constitute the grounding path 5 for electrically grounding the ring gear 33 f.

Both a meshing part 4 d between the gears 33 e and 33 f and a meshing part 4 f between the gears 33 h and 33 i are electrically resistive. Further, while the gear set 3 may further have a spline coupling 4 a for coupling with the rotor shaft 1 b and output couplings 4 b, 4 c for respectively coupling with wheel shafts 3 d′, the couplings 4 a, 4 b, 4 c are also resistive.

As the connection member 5 f is electrically connected with the ring gear 33 g, at least the meshing parts 4 d, 4 e and the coupling 4 a, as electrically resistive elements, are interposed between the rotor shaft 1 b and the connection member 5 f. Thus the resistance ZR in its equivalent circuit shown in FIG. 4 is made relatively higher. Therefore the noise voltage V2 is reduced as described above and then noise radiation is suppressed.

Any various modifications would occur. FIG. 9 illustrates one of such modifications. In this illustration, a resistor R is connected in series in the input shaft 33 b. As the resistor R increases an electric resistance on the line of the input shaft 33 b and the counter shaft CS interposed between brushes B and a rotor shaft (not shown), noise flowing toward the drive shaft DS is suppressed. Location of the resistor R may be alternatively changed.

To ground a part of the gear set, any means other than the brushes 5 a and the connection member 5 f may be alternatively used as far as one or more electrically resistive elements are electrically interposed between the motor and the means.

The teachings as described above can be also applied to continuously variable transmissions. Further the teachings can be also applied to so-called hybrid vehicles, fuel cell vehicles, or any other vehicles where noise radiation is concerned.

Although the invention has been described above by reference to certain exemplary embodiments of the invention, the invention is not limited to the exemplary embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art, in light of the above teachings.

INDUSTRIAL APPLICABILITY

A power transmission device applicable to electric vehicles, which suppresses noise propagation from an inverter through a power transmission device to drive shafts and suspensions, thereby reducing noise radiation to the exterior. 

1. A power transmission device for a vehicle comprising: a motor including a rotor shaft and a stator having an electromagnetic coil; an inverter configured to generate an alternating current, the inverter being connected with the coil to controllably rotate the rotor shaft relative to the stator; a gear set including an input shaft coupled with and rotated by the rotor shaft, an output shaft and gears so meshed as to transmit torque of the input shaft to the output shaft; and a grounding path electrically connecting a part of the gear set with a body of the vehicle, the part being so disposed as to have the input shaft electrically interposed between the part and the rotor shaft.
 2. The power transmission device of claim 1, wherein the part is so disposed as to have the input shaft and one or more meshing pairs of the gears electrically interposed between the part and the rotor shaft.
 3. The power transmission device of claim 1, further comprising: a coupling drivingly interposed between the rotor shaft and the input shaft, the coupling being electrically interposed between the part and the rotor shaft.
 4. The power transmission device of claim 1, further comprising: one or more counter shafts intervening between the input shaft and the output shaft; and a brush in slidable contact with one selected from the group consisting of the input shaft, the counter shafts and the output shaft and connected with the grounding path so as to electrically connect the part of the gear set with the body of the vehicle.
 5. The power transmission device of claim 4, further comprising: a casing carrying the gear set and being electrically connected with the grounding path; and a lead line electrically connected with the brush and the casing.
 6. The power transmission device of claim 4, further comprising: an oil seal intervening between the gear set and the brush.
 7. The power transmission device of claim 5, further comprising: a brush chamber housing the brush; and a breather communicating the brush chamber with an outside of the brush chamber.
 8. The power transmission device of claim 1, wherein the part is an end portion of any of the shafts.
 9. The power transmission device of claim 1, wherein the gear set includes a planetary gear set having a sun gear, pinions meshing with the sun gear, and a ring gear meshing with the pinions, and the part is the ring gear. 